This application is a continuation of International Application No. PCT/JP02/02470, filed Mar. 15, 2002, which claims the benefit of Japanese Patent Application No. 080505/2001, filed Mar. 21, 2001.
1. Field of the Invention
This invention relates to a drive circuit to be used in an active matrix type light-emitting element array for driving and controlling an array of emission type elements such as organic and inorganic electroluminescent (to be referred to as xe2x80x9cELxe2x80x9d hereinafterxe2x80x9d) elements or light-emitting diodes (to be referred to as xe2x80x9cLEDxe2x80x9d hereinafter) and also to an active matrix type display panel realized by using such a drive circuit.
2. Related Background Art
Display devices adapted to display characters and images by means of a dot matrix formed by arranging light-emitting elements such as organic or inorganic EL elements or LEDs are currently popularly being used in television sets, mobile terminals and other applications.
Particularly, display devices comprising emission type elements are attracting attention because, unlike display devices utilizing liquid crystal, they have a number of advantages including that they do not require a backlight for illumination and provide a wide view angle. Above all, display devices referred to as active matrix type devices that are realized by combining transistors and light-emitting elements and adapted to be operated in a drive mode referred to as static drive have been drawing attention because they provide remarkable advantages including high brightness, high contrast and high definition if compared with display devices that operate on a time division drive basis in a simple matrix drove mode.
FIG. 8 of the accompanying drawings is quoted from Preliminary Papers xe2x80x9cEurodisplay xe2x80x9890xe2x80x9d for Autumn Convention 1990, pp. 216-219, published by Society for Information Display. It illustrates a known display circuit of the type under consideration. More specifically, it shows a light-emitting element drive circuit of an active matrix type display device comprising EL elements as light-emitting elements.
As seen from FIG. 8, when the scan line 36 that is connected to the gate of transistor 35 of the drive circuit is selected and activated, the transistor 35 becomes ON and a signal is written in capacitor 38 from the data line 37 connected to the transistor 35. The capacitor 38 determines the voltage between the gate and the source of transistor 41. When the scan line 36 is no longer selected and the transistor 35 becomes OFF, the voltage between the opposite ends of the capacitor 38 is held unchanged until the scan line 36 is selected in the next cycle and the transistor 41 is held ON during that period.
As the transistor 41 becomes ON, an electric current flows from power supply electrode 39 to common electrode 42 by way of EL element 40 and the drain/source of the transistor 41 to drive the organic EL element 40 to emit light.
Generally speaking, for the display terminal of a computer, the monitor screen of a personal computer or the display screen of a television set to display a moving image, it is desirable that each pixel can change the brightness so as to display gradation. As far as organic EL elements are concerned, known systems that have hitherto been used to provide displayed images with gradation include the analog gradation system, the area gradation system and the time gradation system.
The analog gradation system is designed to control the brightness of emitted light of an organic EL element as a function of the quantity of the electric current flowing through the organic EL element. If a thin film transistor (to be referred to as xe2x80x9cTFTxe2x80x9d hereinafter) is used as switching element for supplying the electric current, a control signal is applied as gate voltage according to a video signal so as to control the conductance of the switching element by using a rising region (to be referred to as xe2x80x9csaturated regionxe2x80x9d here for the sake of convenience) of the source current characteristic (Vg-Is characteristic) relative to the gate voltage.
Then, it is necessary to make the gamma (xcex3) characteristic of the video signal change according to the brightnessxe2x80x94voltage characteristic of the organic EL element.
Currently available TFTs include those of the amorphous silicon (a-Si) type and those of the polysilicon (polycrystalline silicon) type (p-Si), of which polycrystalline silicon TFTs are in the mainstream because they show a high mobility and can be downsized in addition to that the process of manufacturing polycrystalline silicon TFTs can be conducted at low temperature due to the recent advancement of laser processing technology. However, generally, polycrystalline silicon TFTs are apt to be affected by the crystal grain boundaries thereof and their electric characteristics can vary remarkably particularly in the saturated region. In other words, even if a uniform video signal voltage is applied to the pixels of the display device, an uneven image can be displayed.
Furthermore, most TFTs are currently being used as switching elements. More specifically, they are adapted to be used in a linearly operating region where the drain current changes proportionally relative to the source voltage when a gate voltage that is considerably higher than the threshold voltage of the transistor is applied so that they are not significantly affected by the varying electric characteristics in the saturated region. However, if polysilicon TFTs are operated in the saturated region in order to adopt the analog gradation system, the display performance of the display device can become unstable as the operation of the TFTs are affected by the varying electric characteristics.
When, for instance, the organic EL element 40 is driven by the TFT circuit to display analog gradation in FIG. 8, the voltage applied between the gate and the source of the transistor 41 is slightly higher than the threshold voltage (Vth) of the transistor. FIG. 9 is a graph illustrating the Vg-Is characteristics of different transistors. The transistors are adapted to utilize the part of the characteristic curve where the source current rises as the gate voltage increases (or the saturated region). However, if the gate voltagexe2x80x94source current characteristic (Vg-Is characteristic) varies as shown in FIG. 9 (or the threshold voltage Vth of the transistor varies), the electric current that flows through the transistor 41 can also vary as indicated by IA (intersection of the curve of a solid line and VA) and IB (intersection of the curve of a broken line and VA) even if a constant gate voltage VA is applied to the gate electrode of the transistor 41 in FIG. 8. Additionally, the brightness of light emitted when a constant voltage is applied may vary depending on the manufacturing process that can involve problems such as film thickness distribution of an organic layer. Such variances are particularly significant when brightness is related to providing gradation. Referring to FIG. 8 again, the part surrounded by dotted lines 43 indicates a region that is apt to produce such variances. Then, organic EL elements 40 that are supposed to show a same level of brightness when a same voltage is applied can actually show different levels of brightness. Such variances in brightness can degrade the quality of the displayed image.
On the other hand, the area gradation system is proposed in AM-LCD2000, AM3-1. It is a system of dividing each pixel into a plurality of sub-pixels so that each sub-pixel can be turned ON and OFF and gradation may be defined by the total area of the pixels that are ON.
With this mode of utilizing organic EL elements, TFTs are used as switching elements so that a gate voltage that is much higher than the threshold voltage is applied to exploit a region of the characteristic curve where the drain voltage is proportional to the source voltage (or the linear region) in order to avoid variances in the TFT characteristic and stabilize the light-emitting characteristic. However, this gradation mode can provide only digital gradation that depends on the dividing manner for the display area and the number of sub-pixels has to be increased by reducing the area of each sub-pixel when raising the number of gradations. Even if transistors are downsized by using polycrystalline silicon TFTs, the area of the transistor arranged in each pixel comes to occupy the corresponding light-emitting area to a large extent to consequently reduce the aperture ratio of the pixel so that by turn the brightness of the entire display panel is inevitably reduced. In other words, the gradation is a tradeoff for the aperture ratio and therefore it is difficult to improve the gradation. Additionally, the density of the drive current flowing through an organic EL element may have to be raised to achieve a desired level of brightness to consequently raise the drive voltage of the element and reduce the service life of the element.
Finally, the time gradation system is a system of controlling the gradation by way of the ON time period of each organic EL element as reported in SID 2000 DIGEST 36.1 (pp. 912-915). However, the TFTs of the display panel have to be driven to operate in a linear region as in the case of the area gradation system in order to minimize the variances in the TFT characteristic so that the problem of a high power supply voltage to be applied to the drive circuit and a high overall power consumption rate remains unsolved.
Additionally, the time gradation system is a complicated system for driving a display device. Currently, for ordinary picture signals transmitted to display devices, brightness signals of three primary colors of RGB are output in the form of analog signals. In the case of video signals, signals are produced by decoding composite signals or Y/C signals into RGB brightness signals. The analog signals need to be changed into PWM signals that are time amplitude signals. For this purpose, as shown in FIG. 10, an AD converter, an image memory, a PWM signal converter circuit and an MPU for controlling them are required.
Furthermore, with the time gradation system, a pulse voltage has to be applied for a very short period of time to each element that is provided with matrix wiring. Therefore, it is necessary to reduce the electric resistance of the matrix wiring system in the display panel. Then, the display panel has to be so designed as to use a low resistance material for the wires and raise the thickness of the wires in order to reduce the electric resistance thereof.
While the analog gradation system requires only a signal amplifying circuit for changing the signal level of RGB analog signals to the brightness signal level that matches the display elements on the display panel as shown in FIG. 11, the time gradation system requires a complex drive system as described above, which by turn raises the power consumption level and the cost of manufacturing the elements. Thus, the time gradation system is accompanied by a number of problems including not only those relating to the performance of the display device but also those relating to the drive system.
However, if the analog gradation system is adopted, the individual transistors can show respective threshold voltages (Vth) that vary from transistor to transistor to a large extent, as mentioned above. Then the output current can also show variances to consequently give rise to variances in the brightness of emitted light.
Variances of the threshold voltage will be briefly discussed below.
As shown in FIG. 8, a TFT for driving an EL element operates as part of a source follower circuit from the circuit point of view. In the source follower circuit, the drain of the TFT is connected to power source Vdd and the gate operates as input terminal, while the source operates as output terminal. Thus, the EL element is arranged between the source of the TFT and the Vss (GND) and an electric current flows through it. If the source terminal voltage is Vout and the gate input voltage is Vin,
Vout=Vinxe2x88x92Vos,
where Vos is the offset voltage generated between the gate and the source.
Generally, if the electric current that flows to the source terminal is Iout, Vos is expressed by
Vos=Vth+{square root over ( )} (Iout/xcex2),
where
xcex2=(1/2)xc3x97xcexcxc3x97Coxxc3x97(W/L),
where xcexc represents the mobility and Cox, W and L respectively represent the gate oxide film capacitance, the gate width and the gate length of the TFT.
As may be clear from the above description, in a source follower circuit comprising TFTs, each individual TFT has its own offset voltage Vos that is specific to it and causes variances in the threshold voltage Vth of transistor. Therefore, it is desired to eliminate the influence of offset voltage and provide a stable output characteristic curve from the viewpoint of driving organic EL elements by means of TFTs with the analog system.
In view of the above identified circumstances, it is therefore the object of the present invention to provide a drive circuit of an active matrix type light-emitting element array that can cancel variances in the signal to be applied to light-emitting elements so as to improve the response speed of the light-emitting element array when a TFT realized using polycrystalline silicon and showing a characteristic that is subject to variance is employed and also provide an active matrix type display panel using such a drive circuit.
In an aspect of the invention, the above object is achieved by providing a drive circuit to be used in an active matrix type light-emitting element array comprising scan lines and signal lines arranged on a substrate to form a matrix and unit pixels formed near the respective crossings of the scan lines and the signal lines, each unit pixel including a light-emitting element and a plurality of thin film transistors each having a source electrode, a gate electrode and a drain electrode, the drive circuit comprising:
a first circuit section including a first thin film transistor (M1) having a gate electrode connected to a scan line, a source electrode connected to a signal line and a drain electrode;
a second circuit section including a light-emitting element having an electrode connected to a first power source and a second thin film transistor (M2) having a gate electrode, a source electrode connected to a second power source and a drain electrode connected to another electrode of the light-emitting element, hence the light-emitting element being connected in series to the second thin film transistor; and
a third circuit section including a third thin film transistor (M3) having a source electrode connected to a reference power source and a drain electrode connected to the gate electrode of the second thin film transistor;
the drain electrode of the first thin film transistor being connected to the gate electrode of the second thin film transistor by way of a memory capacitance (C1);
the drain electrodes of the first and second thin film transistors being commonly connected.
Typically, the voltage of the reference power source is higher than the threshold voltage of the second thin film transistor and lower than the light emission threshold voltage of the light-emitting element.
A drive circuit having a configuration as defined above may further comprise a fourth circuit section including a fourth thin film transistor having a source electrode connected to a reset voltage and a drain electrode connected commonly to the input terminal of the light-emitting element.
This arrangement provides a functional feature of forcibly terminating the light-emitting state of the light-emitting element by turning on the fourth transistor particularly in a field period.
In another aspect of the invention, there is provided an active matrix type display device comprising a plurality of pixel sections arranged in the form of a matrix, the pixel sections respectively having the above drive circuits and the light-emitting elements.